The xylene isomers are important intermediates which find wide and varied application in chemical syntheses. Para-xylene is a feedstock for terephthalic acid which is used in the manufacture of synthetic textile fibers and resins. Meta-xylene is used in the manufacture of plasticizers, azo dyes, wood preservers, etc. Ortho-xylene is feedstock for phthalic anhydride production.
A typical C8-aromatics stream containing all of the xylene isomers and ethylbenzene is not readily separated into individual components due to their close boiling points. Ortho-xylene often is separated by fractional distillation, but requires a tower usually having 100-150 trays with high reflux ratios. Ethylbenzene can be separated by superfractionation with even more difficulty, but this generally is impractical. Para-xylene/meta-xylene separation by distillation is commercially impossible. Para-xylene has been separated by crystallization for some time, and adsorptive separation as disclosed in U.S. Pat. No. 2,985,589 has advanced to dominate commercial production.
Meta-xylene adsorptive separation, as disclosed in U.S. Pat. No. 3,773,846, U.S. Pat. No. 4,306,107, U.S. Pat. No. 4,326,092 and U.S. Pat. No. 5,900,523, also has been applied commercially. One issue in commercialization is that the presence of a significant concentration of ortho-xylene in the feed to meta-xylene adsorptive separation significantly increases equipment sizes and utility costs. This is not a problem if ortho-xylene is recovered by fractionation, but is important if the usual concentration of ortho-xylene is present in the feedstock to meta-xylene recovery.